Electrolytic polishing



Patented Oct. 8, 1946 ELECTROLYTIC POLISHING Clements Batcheller, Glens Falls, N. Y.

No Drawing. Application January 7, 1941, Serial No. 373,461

3 Claims. (Cl. 204140) My invention relates to the electro-chemical treatment of metals and particularly to the treatment of stainless alloys containing nickel,

' or both chromium and nickel. such, for example,

as the chrome-nickel steels.

The electrochemical polishing of metals is a relatively new art which is being developed in an attempt to find methods of surface polishing which are cheaper and better than mechanical bufllng. Various acid electrolytes have been su gested in which the steel to be polished is treated as the anode. Some of these include expensive ingredients and none of them, so far as I am aware, has any unusual throwing power. The cathode must be specially shaped and maintained in carefully spaced relation to the anode if uniform results are to be attained so that it is practically imposibble to polish irregularly formed objects. Furthermore, large area sheet or strip products cannot be uniformly polished because disfiguring streaks and pits are formed in the polished surface for reasons which, at present, are not completely understood.

The principal objects of my invention therefore are to provide a process for the electrochemical treatment of metals which is economical in first cost because of the character of the electrolyte and the simple equipment required, which is cheap to operate because of the long life of the electrolyte, which will uniformly treat irregularly shaped metal objects without the necessity of special cathodes, and which may be used to polish the surface of metal alloys of the group consisting of Monel metal and the chromenickel stainless steels.

As an electrolyte I prefer to use a solution containing the following elements in about the proportions set forth:

Percent by weight Water 35-55 Chromic a 2-20 Free sulphuric acid (1.84 sp. gr.) 15-55 sulphates or bisulphates and impurities- Balance 2 phate of the chromate or dichromate-forming element.

Instead of chromic acid, vanadic or metavanadic acid may be added directly to the bath or formed therein by adding a vanadate 0: metavanadate of ammonium, potassium or sodium. These acids and salts are not only expensive but sometimes diflicult to obtain commercially, and for these reasons I prefer not to use them.

Instead of chromic acid, manganic or permanganic acid may be formed in the bath by the addition thereto of any of the manganates or permanganates of the alkali metals of group I or any of the manganates or permanganates of the metals of group II. The stability of the electrolytes containing these last mentioned acids is not as high as with the other acids mentioned and therefore I prefer to use chromic acid formed as a reaction product by the addition of a chromate or dichromate to the bath.

The electrolyte is preferably contained in a tank of suitable size lined throughout with tellurium lead which may be used as the cathode. Where it is desired'to give the metal a. highly lustrous polish, the bath is heated to a temperature in excess of about F., and preferably between about F.-and the boiling point, by any suitable means such,-for example, as heating tubes or pipes immersed therein. The metal to be treated is simply immersed in the electrolyte as anode and treated at a current density of preferably about 1 /2 to about 15 amperes per square inch of anode. Only a very short period of treatment is necessary to produce a high, uniform polish thereon at 15 amperes. For example, small articles .such as tubes, rods. watch cases and other regularly or irregularly shaped objects formed of chrome-nickel steel, or having some portions formed of chrome-nickel steel and other portions formed of "Monel metal, may

be immersed in any position in the tank and I highly polished in from 20 to 60 seconds-of immersion time. Because of the very short immersion time required to polish, wire, rod or strip products may be run continuously through a bath of proper length at a high rate of speed and small area products may be treated consecutively or in small groups for about a minute or less. Moreover, in such case, although the current density per square inch of anode is comparatively high the total current flowing need not be unreasonably great because of the comparatively small area of the anode.

Processes of the prior art using an electrolyte of orthophosphoric acid or a mixture of ortho- 3 phosphoric acid and sulphuric acid with a current density of about 1 ampere per square inch 01' anode require about minutes of immersion time to polish 18-8 stainless steel. Furthermore. the average loss of metal from the surface of the object during this treatment is from 2.5% to 5.0%

While I prefer to use a current density of from 1% to amperes per square inch or anode surface where a highly polished finish is desired, much higher current densities, even up to 30 amperes, may be employed where adequate i'acilities can be provided for fast handling of the products during the treatment.

I also wish to point out that. in general, the higher temperatures produce the better polishes.

Straight chromium stainless steels or other stainless steels which do not contain appreciable quantities of nickel do not acquire the same extremely bright polish as the nickel-containing steels.

In other words, nickel in quantities of 2% or more, by weight, seems to be an essential ingredient oi the steel if an extremely high reflecting luster is to be applied thereto.

What I claim is:

1. The method of polishing the surface of a metal alloy of the group consisting of "MoneP metal and the chrome-nicke1 stainless steels, which comprises subjecting the alloy to electrolytic action as anode at a current density of from i about it to about 30 amperes per square inch and in an electrolyte containing a plurality oi ingredients of which the following, in the percentages by weight stated, are the only necessary ingredients, iree sulphuric acid from about 15% to about a second acid selected irom the group consisting chromic, vanadic, metavanadic, manganic and permanganic from about 2% to about 20%. and the balance water; the temperature of said electrolyte being in execess of about F.

2. The method of brightening and imparting a high polish to the surface of a chrome-nickel stainless steel which comprise subjecting the steel to electrolytic action as anode at a current density of from. about 1 /2 to about 15 amperes per square inch and in an electrolyte containing a plurality of ingredients in which the following, in the percentages by weight stated, are the only necessary ingredients, iree sulphuric acid from about 15% to about 55%, a second acid selected from the group consisting of chromic, vanadic, metavanadic, manganic and. permanganic from about 2% to about 20%. and the balance water; the temperature of said electrolyte being in excess of about 160 F.

3. The method of polishing the surface of a stainless steel alloy containing chromium and at least 2%, by weight, of nickel, which comprises subjecting the alloy to electrolytic treatment as anode at a current density of from about 1 to about 15 amperes per square inch and in an electrolyte containing, 'by weight, from about 15% to about 55% of free sulphuric acid, from about 2% to about 20% of chromic acid, and the balance being substantially water with minor quantities oi. sulphates, bisulphates and impurities; said electrolyte being maintained at a temperature between about 160 F., and the boiling point during said treatment. 7

CLEMENTS BATCHELLER. 

